OBJECTIVE: Genetically engineered bacterial strains were developed and used as live, oral vaccines to immunize against mucosal SIV infection in rhesus macaques. RESULTS We developed recombinant Salmonella typhimurium strains that express the SIV p27 antigen from a chromosomally-inserted expression construct. Live Salmonella were delivered by intragastric intubation of juvenile rhesus macaques and immune responses to the bacterial vector and the recombinant protein were characterized. In parallel animals received intramuscular immunization with purified, recombinant p27 antigen in polyphosphazene adjuvant. Intragastric immunization with recombinant bacteria elicited strong lymphoproliferative responses to Salmonella and to the viral protein without generating a measurable serum immunoglobulin response. Intramuscular immunization with soluble viral protein failed to elicit lymphoproliferative responses but generated high levels of serum antibody. Intragastric immunization did not boost the serum immunoglobulin response initiated by prior intramuscular immunization. Likewise, intramuscular boosting did not increase levels of lymphoproliferative responses to viral antigen. These studies provide insight into the separate and distinct mechanisms for peripheral and mucosal immune responses in the rhesus macaque. Additionally, these immunization strategies are important for future efforts to map the pattern and rates for CD4 helper T cell depletion during acute viral infection of macaques. FUTURE DIRECTIONS Complete the analysis of vaccinated and challenged animals and extend the utility of Salmonella vaccines as tools for basic immunology studies in the SIV-infected macaque. Develop Salmonella as a delivery vehicle for DNA immunization of gut mucosal tissues. KEY WORDS AIDS, vaccine, mucosal, recombinant bacteria, Salmonella typhimurium AI36643 and RR00167